Erosion is the wastage of objects by the impingement of hard particles traveling in a gaseous or liquid fluid. Several factors which control the rate of wastage of a given surface are:
the relative hardness of the erosive media and the surface being eroded; PA1 the temperature, size, shape, velocity, and angle of impingement of the eroding media; and PA1 the smoothness, ductility, and integrity (lack of porosity) of the surface being eroded.
Other factors may also be considered. In instances where multi-phase materials are specified to enhance erosion resistance (such as cemented carbides), the size and shape of the second phase carbide particles also have a direct bearing on the erosion resistance of a given surface. Because of the complexity and interrelationship of all of these factors, it often is not possible to predict whether the erosion resistance of a given surface will be good under a specific set of erosive conditions.
The present invention comprehends a material which has much better erosion resistance under a widely varying set of erosive conditions than many materials which are customarily specified for use in erosive environments. The subject material is a cemented carbide comprised of niobium carbide in a metal matrix, along with a process for applying the material to a metal substrate. The erosion resistance of such material has been evaluated as a coating which has been applied onto the surface of steel specimens.
The subject invention was made during the course of a Small Business Innovative Research grant to Manhattan Turbine Corporation by the Department of Energy. This grant was awarded following submission of a proposal in response to a solicitation requesting novel solutions for the control of erosion in fluidized bed combustors.
The combustion of coal in a fluidized bed combustor results in the formation of large quantities of ash, which is comprised principally of silica, and, as such, is highly erosive. This results in the degradation of heat exchanger tubing in the fluidized bed combustor. The specific degradation is of two primary types: erosion/corrosion of "in-bed" tubing in the 450.degree. C. temperature range; and erosion of tubing in the 300.degree. C. range at the base of the waterwall.
A large number of protective coating systems have been used by various fluidized bed combustor manufacturers to slow the rate of wastage in heat exchanger tubing. In most instances, the coatings applied have not extended the life of the tubing sufficiently to be considered cost effective.